iio: fix drivers that use 0 as a valid IRQ in client->irq (part 2)

Since commit dab472eb ("i2c / ACPI: Use 0 to indicate that
device does not have interrupt assigned") 0 is not a valid i2c
client irq anymore, so change all driver's checks accordingly.

The same issue occurs when the device is instantiated via device tree
with no IRQ, or from the i2c sysfs interface, even before the patch
above.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>